The present invention relates, in general, to amplification circuits and, more particularly, to radio frequency amplifiers.
In a radio frequency (RF) amplifier, especially in a power RF amplifier or a high linearity RF amplifier, the direct current (DC) or quiescent current biasing point is a critical design parameter. The quiescent current of an RF amplifier has significant effects on the characteristics and performance, e.g., linearity, signal distortion, power efficiency, etc., of the amplifier. The optimal DC biasing point of a transistor in an amplifier depends on the application of the amplifier and the characteristics of the transistor, which are affected by temperature variation, process variation, history of the transistor, etc. The DC biasing point changes as the transistor characteristics vary.
One technique for maintaining the optimal DC biasing point is referred to as self bias technique, in which the transistor to be biased also operates as a DC amplifier and the amplified DC signal is used as a feedback to adjust the bias. The self bias technique adversely affects the RF performance of the amplifier and is inappropriate for high performance RF amplifiers. The widely practiced active bias trimming technique can adjust DC biasing points according to process variations of a device, but it cannot adjust the DC biasing points in response to temperature variations and the history of the device. Resetting the biasing points after burning in the device can often lessen but not eliminate the problem of DC biasing point drifting as the device ages. Further, the burning in process is time consuming and costly. A large DC resistance connected in series with the emitter of a bipolar transistor can reduce the temperature sensitivity of the transistor. However, the voltage drop across and the power loss in the large resistance adversely affect the performance of the amplifier that includes the bipolar transistor. Another technique uses a microprocessor controlled active bias control circuit to periodically reset the DC bias points. This technique is complicated and expensive.
Accordingly, it would be advantageous to have an active bias compensation circuit and a method for adjusting a quiescent current so that an optimal quiescent current biasing point is maintained over temperature variation, process variation, and life time of an amplifier. It is desirable for the circuit to be simple and space efficient. It is also desirable for the method to be power efficient and cost efficient. It would be of further advantage for the circuit and the method to be compatible with high performance wide band linear RF amplifiers